In general, a computerized scale of the foregoing type includes a series (e.g., ten) of so-called weighing buckets, each associated with an underlying load cell or other means for producing an electrical signal representative of the weight of product in the bucket. Located above each weighing bucket is a holding bucket which contains a quantity of the product to be packaged. During each cycle, each empty weighing bucket is filled with product by momentarily opening the overlying holding bucket and allowing the product to fall into the weighing bucket. The weight of the product dropped into each weighing bucket is substantially less than the total weight of product which subsequently is placed in each package by the packaging machine.
After all of the weighing buckets have been filled, microprocessor-based control circuitry responds to the weight signals produced by the different load cells, adds the weights in various combinations of weighing buckets and then selects the particular combination of buckets that meets the minimum statistical weight for the package to be filled while providing the least excess weight. The weighing buckets of that particular combination are emptied and the product therein is delivered to the packaging machine to be deposited in the package. Only those weighing buckets previously emptied are refilled by dumping from their respective holding buckets during the succeeding cycle.
Because of product buildup and other well-known problems, the tare weights of the weighing buckets may change over a number of weighing cycles. In order to compensate for dynamic changes in the tare weights of the weighing buckets, new tare weights must be periodically calculated.
One example of a method to accomplish a tare weight calculation is disclosed in U.S. patent application Ser. No. 742,553 to Moran, assigned to the assignee of the present invention. In the Moran application, the sequence of weighing cycles is periodically interrupted in order that a tare cycle may be executed. During the tare cycle, all the weighing buckets of the weighing machine which were emptied in the last weighing cycle are weighed in their empty state. The new values for the tare weights are stored and the old values are discarded. By introducing a tare cycle between successive weighing cycles, all weighing buckets are available in each weighing cycle for finding the best combination.
After repeated weighing cycles, it may be required to replace some parts of the apparatus such as the load cells and/or weighing buckets. After maintenance work has been completed, the electrical voltage indicative of the weight of a bucket may be significantly different than the voltage before the repair. For example, each new load cell may not have exactly the same electrical characteristics as the load cell it replaces; therefore, resolution of the net weight placed in the bucket during a weighing cycle may be decreased if the voltage signal from the load cell for an empty bucket is significantly more than it was previously. Manual calibration of the system after each occurrence of a part replacement requires considerable system "down time" and as such represents a significant expense as measured in lost packaging time and reduced productivity.
In addition to lost resolution resulting from component changes, dynamic changes in the tare weights over many weighing cycles may substantially affect the dynamic range, and therefore the resolution, of the load cells and their associated electronics. For example, after repeated weighing and tare cycles for a particular product, a weighing bucket may build up a residue of product which increases the tare weight of the bucket beyond an acceptable value for maintaining good resolution of net weight. In addition, lesser changes in the effective tare weight may be caused by drifting output voltages as a result of the exposure of the electronics to extreme ambient temperatures. For some products and for some environments, these changes in the tare weights may be significant.
As voltage values for the tare weights of the weighing buckets increase, the resolution of the net weights of the products by the computerized scale decreases. With decreased resolution, the scale is less able to determine the best combination of weights. Although the effect of the reduced resolution may be insignificant over a few packaging cycles, the cumulative effect of the reduced resolution is great when considered over a longer work cycle such as a few days, weeks or months.